Bus bars and bus bar modules have conventionally been used in electric connection. For example, bus bars and bus bar modules are used in hybrid automobiles and electric automobiles where pulse width modulation (PWM: Pulse Width Modulation) driving control using particularly high-frequency current is performed.
An example of the bus bar used in the system of a hybrid automobile will be described here with reference to PTL 1 and 2. In the example in PTL 1 and 2, bus bars are used for electric connection between the motor and motor inverter, and between the generator and generator inverter, and also for electric power lines within the inverter unit.
Generally, high-frequency current flowing among the motor, generator, and invertor, includes high-frequency wave components due to switching that are as high as several kHz, besides the basic sine wave and DC component. Such high-frequency wave components are induced by eddy current within the bus bar conductor. In the example in PTL 1, the current flow in a manner concentrated beneath the surface skin of a bus bar 101 as illustrated in FIG. 11. The skin depth thereof is obtained as δ=(ρ/πfμ)1/2 from the frequency f of the current and the conductor material of the bus bar 101 constructed of rectangular wire. This lowers the current density flowing thorough the conductor, so the effective resistance within the conductor increases, consequently being manifested as eddy current loss. The eddy current loss is proportionate to the current frequency f squared, so the AC current generated by PWM exhibits marked eddy current loss due to the current with a significantly large high-frequency current flowing through the bus bar 101. Now, FIG. 10 is a graph illustrating the relationship between the frequency of the driving current and the skin depth, for each of various types of conductor materials.
Copper plate-shaped bus bars having a large surface area are used in motors using large currents with high voltage as described in PTL 1, in order to suppress the above-described eddy current loss due to high-frequency waves, and for thermal dissipation. However, the prime power, which is the basic sine waves with relatively low frequencies, and DC component, also flows through the bus bar. Accordingly, forming the bus bar as a flat plate to reduce the cross-sectional area in order to suppress the high-frequency wave components increases the effective resistance for the current handling the prime power, resulting in an increase in so-called copper loss (or iron loss in a case where the material is iron). Further, metal plates formed of material such as copper, which are plate-shaped with a certain degree of thickness, have rigidity to a certain extent, so the forming and wiring implementation thereof is not easy. Accordingly, how to reduce overall transmission loss in bus bars which transmit current in which low-frequency waves through high-frequency waves coexist is an issue.
The high-frequency wave components accompanying PWM also induce reactive voltage proportionate to the product of the inductance and frequency (V∝f·L) in the bus bar, so the faster the switching is, the greater the breakdown voltage of the output stage element of the invertor has to be to deal with surges thereof. Accordingly, the floating inductance of the bus bar or bus bar module is preferably as small as possible.
On the other hand, an assembled rectangular wire where multiple relatively fine rectangular wires have been assembled makes up the bus bar. PTL 2 asserts that this reduces manufacturing costs, enables forming of complicated shapes, and further enables eddy current loss to be suppressed by splitting the current among multiple rectangular wires. According to the description in PTL 2, the wire diameter is reduced to (1/number of coil wires) in a case of configuring the bus bar using multiple rectangular wires, as compared with forming the bus bar of a flat plate. The description states that this suppresses the eddy current loss which is said to be proportionate to the line width squared, and consequently suppresses eddy current loss over the entire bus bar. The description also states that increasing the number of lines and reducing the line diameter of each also enables the loops of the eddy currents flowing through the cross-section to be reduced, and that eddy current loss can be further reduced.
However, the skin effect still remains in a configuration where a bus bar 102 is made up of multiple rectangular wires arrayed sideways in parallel, as illustrated in FIG. 12A. That is to say, assuming that externally-supplied high-frequency current is uniformly split among the rectangular wires making up the bus bar 102, it can be understood that there will be magnetic flux lines surrounding the rectangular wires on the inner side, when considering the high-frequency wave magnetic flux lines excited thereby, as illustrated in FIG. 12B. The two rectangular wires on the outer sides of the magnetic flux lines form a large closed loop, due to being connected by both terminals of the bus bar 102, and the AC magnetic flux lines traverse this loop. The effects of the electromagnetic induction in this state create induced electromotive force in the closed loop, resulting in an eddy current. This eddy current added to the externally supplied current assumed earlier is conceivably the current actually flowing. Consequently, even of the bus bar 102 is divided into multiple rectangular wires, the current flows avoiding the rectangular wires on the inner side, and so an uneven flow occurs where the current is concentrated at the rectangular wires on the outer side. As a result thereof, a current distribution still remains that is the same as that due to the skin effect in the bus bar 101 illustrated in FIG. 11 made up of the multiple rectangular wires arrayed sideways in parallel have been formed integrally. This is the same as forming slits in parallel in a bus bar made up of rectangular wires not affecting the current flowing through the bus bar whatsoever.
In the same way, the skin effect remains even if configuring a bus bar 103 out of multiple rectangular wires arrayed in parallel sideways and vertically as illustrated in FIG. 13A. Obtaining the actual current distribution in the same way as in FIG. 12B shows that the existence of the magnetic flux lines passing through the inside of the rectangular wires on the inner side results in the same sort of uneven current flow in the same way as the skin effect, after all. As a result thereof, a current distribution still remains that is the same as that due to the skin effect in the bus bar 101 illustrated in FIG. 11 made up of the multiple rectangular wires arrayed sideways in parallel have been formed integrally.
Thus, it can be understood that the bus bar structure shown in PTL 2 demonstrates no effects of suppressing eddy current loss whatsoever, although ease of forming and wiring implementation is improved. PTL 2 states that eddy current loss is further reduced by twisting the entire assembled multiple rectangular wires. However, it can be understood that this configuration has no influence on the form or distribution of the AC magnetic flux lines in the cross-sectional diagrams illustrated in FIG. 12B and FIG. 13B, and has no effect of reducing eddy current induced by the AC magnetic flux lines nor power loss occurring thereby. Moreover, twisting the entire assembled rectangular wire is equivalent to having internal inductance as in a solenoid coil, leading to unnecessary increase in inductance.